1. Field of the Invention
This invention relates to the construction of mirrors for astronomical or surveillance applications in general and, in particular, to a construction for a large, lightweight mirror.
2. Description of the Prior Art
The use of large, lightweight mirrors for astronomical telescopes and for use in optical systems involved in surveillance applications is well known in the prior art. Historically, large mirrors used for astronomical applications have been manufactured from large monolithic blanks of glass with a reflecting surface supported by a relatively thick substrate to insure that the shape of the reflecting surface is accurately maintained both during the process of fabricating the mirror and after completion. As requirements evolved for increasingly larger surface mirrors with complex (i.e. non-spherical surface shapes), the use of the well-known expedient of increasing the thickness of the mirror blank to insure stability of the reflecting surface became less desirable due to the substantial increase in weight of the mirror's structure. Recent advances in technology have resulted in a need for very large diameter mirrors for both astronomical and surveillance applications. Frequently, the applications require that the mirrors be placed in earth orbit. In view of the foregoing requirements, it is particularly important that the mirrors be as light in weight as possible so as to lessen the payload required to be carried into space and, in the case of terrestrial applications, so that the weight of the mirror does not require excessively complicated support structures and, further, to avoid gravity-induced sag in the shape of the mirror's reflecting surface. In addition, when large lightweight mirrors are used in applications involving systems for changing the directions of orientation of the optics, the mirrors ideally should be of as light weight as possible to permit the adjustment of the mirror's tip or tilt using actuators which may be accurately controlled and which have relatively low output forces. The less the weight of the mirror whose orientation or position must be controlled, the smaller the amount of force that must be applied to change the orientation of the mirror.
In response to the difficulty of handling and fabricating large blanks of glass or other materials used for the manufacture of large surface area mirrors, techniques have been developed for producing lightweight mirror structures which result in relatively rigid reflecting surfaces having high optical tolerances, while reducing the over-all weight of the mirror's structure. Principally these techniques have involved the construction of composite mirrors with a center lightweighted core which is sandwiched between a facesheet and a backsheet which are bonded to the core. Notwithstanding the requirement for the mirror to be as light in weight as possible, the mirror must, while meeting the weight restrictions, have sufficient rigidity and stability so that it can be efficiently controlled and/or otherwise handled, and so that its reflecting surface can be precisely finished to insure optimum optical characteristics. Rigidity is often achieved by increasing the depth of the mirror's core. However, this results in an increase in the overall weight of the mirror structure. Representative patents disclosing prior known constructions for lightweight mirrors include U.S. Pat. No. 4,692,367 which issued on Sept. 8, 1987 and which discloses a construction for a lightweight mirror in which honeycomb panels comprises the core of the mirror, and back plate and face plate members are bonded to the core. U.S. Pat. No. 4,466,700 which issued Aug. 21, 1984 discloses a construction for a lightweight mirror in which still another arrangement for producing a lightweight core is described with the core being sandwiched between front and back plates to produce a lightweight mirror. U.S. Pat. No. 3,644,022 which issued Feb. 22, 1972 discloses a construction for a mirror in which the core is built-up from a plurality of y-shaped members which are jointed together to form a honeycomb structure.
In addition to producing lightweight mirrors by constructing a core of separate elements and bonding a facesheet and a backsheet to the composite core as described in the above-noted patents, the prior art discloses the manufacture of lightweight mirrors from a monolithic block of glass in which a number of cavities are machined into the core to reduce the weight of the core structure. Producing the cavities can be accomplished by physically machining a series of holes through the back surface of the mirror to create a honeycomb core structure which supports a front faceplate section of the core, on which the reflecting surface can be polished. However, use of the foregoing process, i.e. machining "lightweighting" holes from the back of the core toward the faceplate, requires that care be taken to avoid having the machine tool either puncture the faceplate as it approaches the front of the mirror, or leave an excessive amount of glass on the faceplate. In the case where the faceplate is punctured, it is virtually impossible to salvage the mirror structure and the entire mirror structure, together with all work accomplished to the time of damage, must be discarded. When excessive material is left on the faceplate, the inherent weight of the mirror structure is not reduced as much as possible and, in the case where the mirror's position is to be controlled by actuators, additional weight is retained in the facesheet which reduces the positioning performance of the mirror. In addition, the task of machining the core by entering the back of the core and machining toward the faceplate requires that the machining operation be precisely carried out as the coring machine approaches the facesheet to avoid damaging the facesheet. This dictates that additional time be utilized to manufacture the mirror and, for mirrors having very large surface areas, this additional machining time becomes appreciable.